Basic Cabling Testing
Power Sum Values
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Minimizing Crosstalk
Crosstalk problems are minimized by twisting together the two wires in each cable
pair. Twisting the two wires together causes the electromagnetic fields around the
wires to cancel out, leaving virtually no external field to transmit signals to nearby
cable pairs.
You can avoid problems with crosstalk by observing the following precautions
during installation:
• When untwisting cable pairs to install connectors or make connections at
punch-down blocks, make the untwisted sections as short as possible.
• Be attentive when making wiring connections. Wiring errors that cause split
pairs create severe crosstalk problems.
• Do not make sharp bends or kinks in the cable. Check the cable manufacturer’s
specifications for the minimum bend radius.
• Handle LAN cable carefully during installation. Do not step on the cable or
pinch it with tight cable ties.
Power Sum Values
Power sum values, such as PSNEXT, PSELFEXT, and PSACR, show how much a
wire pair is affected by the combined interference from the other pairs in the cable.
Power sum NEXT values show how much a pair is affected by the combined
NEXT from the other pairs. PSELFEXT values show the effects of the combined
FEXT. PSACR values show the ratio of a pair’s attenuation to the combined
NEXT of the other pairs.
Good power sum performance is important for higher speed networks, such as
1000BASE-T, that transmit data in parallel over multiple wire pairs. Though a
cable link may not be installed for use with parallel transmission systems,
verifying a link's power sum performances helps ensure successful upgrades in the
future.
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